i) Field of the Invention
The present invention relates to a discharge lamp to be used for a copy lighting device for information apparatuses such as a facsimile, a copier, an image reader and the like, a lightning bulletin board, a large display device, and the like, a display device using the discharge lamp, and a method for producing the discharge lamp.
ii) Description of the Related Arts
Conventionally, a fluorescent lamp is used as a light source for a copy lighting device of information apparatuses such as a facsimile, a copier, an image reader and the like. For such uses, a small type, a high luminance, a long life and high reliability are required for the lamp. Since the conventional fluorescent lamp is provided with electrodes such as filament electrodes within the tube, the structural limitation imposed by the electrodes is large, and a variety of attempts have been tried for settling problems.
In FIGS. 30a and 30b, for example, there is shown a conventional fluorescent lamp disclosed in proceedings of 1991 annual conference of the Illumination Engineering Institute of Japan. As shown in FIGS. 30a and 30b, the fluorescent lamp 1 comprises a cylindrical glass bulb 2 enclosing rare gases mainly composed of xenon gas therein, a fluorescent substance layer 3 formed on the internal surface of the glass bulb 2, a light output part 4 for emitting the generated light in the glass bulb 2 to the outside, a pair of external electrodes 5a and 5b mounted on the external surface of the glass bulb 2 and extending in the longitudinal direction thereof, and a power source 7 for supplying power between the external electrodes 5a and 5b through lead wires 6a and 6b.
When a voltage is applied between the external electrodes 5a and 5b from the power source 7, a current flows between them due to the electrostatic capacity therebetween and brings about a discharge between them both. By this discharge, UV (ultraviolet) rays are generated within the glass bulb 2, and the generated UV rays excite the fluorescent substance layer 3 formed on the internal surface of the glass bulb 2 to irradiate visible light outside through the light output part 4.
In the conventional fluorescent lamp, the aforementioned various defects due to the presence of the electrodes such as the filament electrodes within the glass bulb 2 can be improved upon. However, the following problems are still present. That is, as shown in FIGS. 30a and 30b, the distance between the electrodes on the opposite side to the light output part 4 is almost the same as the width of the light output part 4, and thus the sufficient electrode area can not be taken. Hence, a sufficient light output can not be obtained. Also, as the charged pressure of the rare gases within the glass bulb 2 is increased, the discharge between the electrodes 5a and 5b becomes unstable, and thus a fringe flicker is caused between the electrodes 5a and 5b. Further, since the distance between the electrodes 5a and 5b is wide, the size of the fringe caused between the electrodes 5a and 5b is wide. That is, due to this fringe, the luminance distribution in the longitudinal direction of the fluorescent lamp is uneven. The uneven luminance distribution brings about a problem in a case where the fluorescent lamp is used for the copy lighting of information apparatuses, where a plurality of fluorescent lamps are arranged to constitute an image display device, or the like.